feat: tag types are now required to be empty

src/core/include/mp-units/framework/dimension_concepts.h

@@ -42,7 +42,7 @@ struct dimension_interface;
  * Satisfied by all dimension types in the library.
  */
 MP_UNITS_EXPORT template<typename T>
-concept Dimension = std::derived_from<T, detail::dimension_interface> && std::is_final_v<T>;
+concept Dimension = std::derived_from<T, detail::dimension_interface> && std::is_empty_v<T> && std::is_final_v<T>;
 
 MP_UNITS_EXPORT template<symbol_text Symbol>
 struct base_dimension;

src/core/include/mp-units/framework/quantity_point_concepts.h

@@ -65,7 +65,7 @@ struct point_origin_interface;
  * Satisfied by either quantity points or by all types derived from `absolute_point_origin` class template.
  */
 MP_UNITS_EXPORT template<typename T>
-concept PointOrigin = std::derived_from<T, detail::point_origin_interface> && std::is_final_v<T>;
+concept PointOrigin = std::derived_from<T, detail::point_origin_interface> && std::is_empty_v<T> && std::is_final_v<T>;
 
 /**
  * @brief A concept matching all quantity point origins for a specified quantity type in the library

src/core/include/mp-units/framework/quantity_spec_concepts.h

@@ -37,7 +37,8 @@ struct quantity_spec_interface_base;
 }
 
 MP_UNITS_EXPORT template<typename T>
-concept QuantitySpec = std::derived_from<T, detail::quantity_spec_interface_base> && std::is_final_v<T>;
+concept QuantitySpec =
+  std::derived_from<T, detail::quantity_spec_interface_base> && std::is_empty_v<T> && std::is_final_v<T>;
 
 MP_UNITS_EXPORT
 #if MP_UNITS_API_NO_CRTP

src/core/include/mp-units/framework/unit_concepts.h

@@ -43,7 +43,7 @@ struct unit_interface;
  * Satisfied by all unit types provided by the library.
  */
 MP_UNITS_EXPORT template<typename T>
-concept Unit = std::derived_from<T, detail::unit_interface> && std::is_final_v<T>;
+concept Unit = std::derived_from<T, detail::unit_interface> && std::is_empty_v<T> && std::is_final_v<T>;
 
 template<Magnitude auto M, Unit U>
   requires(M != magnitude<>{} && M != mag<1>)